Make deselection reasons part of pytest_deselected, not a side channel

AI-authored. I prompted this exploration; the agent (Claude Opus 5 via Claude Code)
wrote the analysis and this text. I reviewed it and I am posting it.

Follow-up to #15035, which makes pytest record why it deselected an item and show it
at -v.

The reason there is side-channeled: _pytest.deselect.deselect_items() stashes it on
the config for the duration of the pytest_deselected call, and pytest's terminal
reporter reads it back. It is private, so a plugin can neither record a reason for its
own deselections nor read the reason for pytest's.

That is the whole point of this issue: the side channel should not become the API.

Why the hook cannot just grow the argument

pytest_deselected is unusual — plugins call it, not only implement it. Calling it
from pytest_collection_modifyitems when you remove items is part of the documented
contract. So reason would be a hookspec argument that:

• no existing caller passes, so implementations can never rely on it;
• can never become required, because requiring it breaks every plugin that supports
  more than one pytest version;
• cannot be told apart, when absent, from "the caller had nothing to say".

pluggy has no way to fill in an argument on behalf of an old caller. This is
pytest-dev/pluggy#170 ("Howto handle hook changes"), where this very hook is named as
the motivating case, and pytest-dev/pluggy#361 proposes the mechanism that would
resolve it: letting a hookspec take part in the call and normalize its arguments.

What would need to happen

1. pluggy grows call control for hookspecs (pytest-dev/pluggy#361) or some other way to
   express "this argument is new; here is what old callers mean".
2. pytest_deselected gains reason: str | None, with the spec supplying None for
   callers that do not pass it.
3. _pytest/deselect.py becomes a thin shim, or goes away; the terminal reporter reads
   the hook argument instead of the stash.
4. The placeholder pytest currently prints for plugin deselections
   (deselected by a plugin, no reason recorded) stops being the normal case.

Alternatives worth weighing before that

• A new hook (pytest_items_deselected(items, reason)) alongside the old one, with
  the old one kept for compatibility. Runs into the "both implementations fire" problem
  from pytest-dev/pluggy#170 and needs a deprecation story anyway.
• A deselection API on Session/Config that plugins call instead of the hook
  (session.deselect(items, reason=...)), with the hook fired internally. This was on
  the table in 2016; doing it properly means retiring pytest_collection_modifyitems
  as the place where selection is decided, which is a much bigger change — see the
  discussion in #13617.

Making the side channel public is not on that list on purpose: it would be a second,
parallel way to pass an argument to a hook, and it would have to be kept alive
regardless of what pluggy ends up offering.

Start by reading the pytest_deselected hook, _pytest/deselect.py, and the pluggy issues #170 and #361 that the issue identifies as prerequisites. Trace how the terminal reporter currently reads the deselection reason. Done means the hook can receive normalized reason values from old and new callers, with the side channel no longer required.